Psychosis and agitation often occur in the course of dementia and are a major source of patient disability and caregiver stress. For the common situation in which extrapyramidal (parkinsonian) motor dysfunction accompanies dementia, there is a therapeutic dilemma since the most frequently used drugs to treat the behavioral problems, neuroleptic antipsychotics, can worsen parkinsonism and have been associated with severe extrapyramidal reactions in some types of dementia. To date, the efficacy and tolerability of the most promising alternative medications to treat psychosis and agitation, namely atypical antipsychotics and cholinesterase inhibitors, have not been tested in patients with a primary dementia selected for coexisting parkinsonism. Furthermore, no study has examined the possible additive benefits or risks of these 2 drug classes when used in combination. Proposed is a multicenter (20 member sites of the Alzheimer's Disease Cooperative Study [ADCS]), randomized, controlled clinical trial in which 120 subjects with a primary dementia (probable Alzheimer's disease [AD] or probable dementia with Lewy bodies [DLB]) and coexisting parkinsonism will be randomized according to a 2 X 2 factorial design to 1 of 4 treatments: quetiapine (QUET; an atypical antipsychotic with a favorable extrapyramidal side effect profile), donepezil (a cholinesterase inhibitor), the combination of QUET + DONEP, or placebo. Each subject participates in the trial for 10 weeks and systematic ratings of behavior, motor function, cognition, adverse events and other outcomes occur at baseline and after 6 and 10 weeks of assigned treatment. This study will provide important information regarding the optimal treatment of psychosis and agitation for patients with the commonly encountered combination of primary dementia and parkinsonism.